U.S. patent application number 12/279089 was filed with the patent office on 2009-01-01 for acoustic telemetry.
This patent application is currently assigned to QINETIQ LIMITED. Invention is credited to Roger Patrick Dalton, Ian Andrew Jamieson, Matthew Waters.
Application Number | 20090003133 12/279089 |
Document ID | / |
Family ID | 36383923 |
Filed Date | 2009-01-01 |
United States Patent
Application |
20090003133 |
Kind Code |
A1 |
Dalton; Roger Patrick ; et
al. |
January 1, 2009 |
Acoustic Telemetry
Abstract
A method of transmitting data acoustically through a tubular
structure, such as a drill string or production tubing in an oil or
gas well, predominantly comprising a series of tubing sections
(1)joined end to end by couplings (2), at least a preponderance of
the tubing sections having an axial length of at least a dimension
X between couplings and at least a preponderance of the couplings
having an axial length of no more than a dimension x, where X is
substantially greater than x. The method comprises propagating
acoustic signals along the structure, between transducers (9,10)
over a distance N of at least 10X, in the form of tone bursts at
least predominantly comprising a selected guided wave mode
(preferably the L(0, 1) mode at low frequency) with a wavelength of
at least 2x, and each burst having a temporal length of
substantially less than 2N/C and preferably no more than 2X/C,
where C is the phase velocity of the selected mode. In this way
interference problems associated with Brillouin scattering in such
structures can be overcome without excessive power consumption.
Inventors: |
Dalton; Roger Patrick;
(Hampshire, GB) ; Jamieson; Ian Andrew; (Fife,
GB) ; Waters; Matthew; (Hampshire, GB) |
Correspondence
Address: |
MCDONNELL BOEHNEN HULBERT & BERGHOFF LLP
300 S. WACKER DRIVE, 32ND FLOOR
CHICAGO
IL
60606
US
|
Assignee: |
QINETIQ LIMITED
|
Family ID: |
36383923 |
Appl. No.: |
12/279089 |
Filed: |
March 20, 2007 |
PCT Filed: |
March 20, 2007 |
PCT NO: |
PCT/GB2007/000970 |
371 Date: |
August 12, 2008 |
Current U.S.
Class: |
367/82 ;
175/40 |
Current CPC
Class: |
E21B 47/16 20130101 |
Class at
Publication: |
367/82 ;
175/40 |
International
Class: |
E21B 47/16 20060101
E21B047/16; B06B 1/00 20060101 B06B001/00 |
Foreign Application Data
Date |
Code |
Application Number |
Mar 22, 2006 |
GB |
0605699.8 |
Claims
1. A method of transmitting data acoustically through a tubular
structure predominantly comprising a series of tubing sections
joined end to end by couplings, at least a preponderance of said
tubing sections having an axial length of at least a dimension X
between couplings and at least a preponderance of said couplings
having an axial length of no more than a dimension x, where X is
substantially greater than x; the method comprising propagating
along the structure, from a first position thereon, acoustic
signals in the form of tone bursts at least predominantly
comprising a selected guided wave mode with a wavelength of at
least 2x, and detecting said signals from a second position on the
structure, where the distance N along the structure between said
first and second positions is at least 10X, and wherein each said
burst has, at least as initially propagated, a temporal length of
no more than a multiple of X/C and substantially less than 2N/C,
where C is the phase velocity of the selected mode.
2. A method according to claim 1 wherein said temporal length is
not substantially more than about 20X/C.
3. A method according to claim 2 wherein said temporal length is
not substantially more than 10X/C.
4. A method according to claim 3 wherein said temporal length is
not substantially more than 5X/C.
5. A method according to claim 4 wherein said temporal length is
not substantially more than 2X/C.
6. A method according to claim 5 wherein said temporal length is
less than 2X/C.
7. A method according to claim 1 wherein the selected guided wave
mode is the L(0,1) mode at low frequency.
8. A method according to claim 1 wherein the structure is a drill
string or production tubing within an oil or gas well.
9. Apparatus for transmitting data acoustically over a distance N
through a tubular structure predominantly comprising a series of
tubing sections joined end to end by couplings, at least a
preponderance of said tubing sections having an axial length of at
least a dimension X between couplings and at least a preponderance
of said couplings having an axial length of no more than a
dimension x, where X is substantially greater than x and N is at
least 10X; the apparatus comprising a transducer for propagating
along the structure, from a first position thereon, acoustic
signals in the form of tone bursts at least predominantly
comprising a selected guided wave mode with a wavelength of at
least 2x, each said burst having, at least as initially propagated,
a temporal length of no more than a multiple of X/C and
substantially less than 2N/C, where C is the phase velocity of the
selected mode, and a transducer for detecting said signals from a
second position on the structure spaced along the structure from
the first by said distance N.
10. Apparatus according to claim 9 wherein said temporal length is
not substantially more than about 20X/C.
11. Apparatus according to claim 10 wherein said temporal length is
not substantially more than 10X/C.
12. Apparatus according to claim 11 wherein said temporal length is
not substantially more than 5X/C.
13. Apparatus according to claim 12 wherein said temporal length is
not substantially more than 2X/C.
14. Apparatus according to claim 13 wherein said temporal length is
less than 2X/C.
15. Apparatus according to claim 9 wherein the selected guided wave
mode is the L(0, 1) mode at low frequency.
16. A tubular structure predominantly comprising a series of tubing
sections joined end to end by couplings, at least a preponderance
of said tubing sections having an axial length of at least a
dimension X between couplings and at least a preponderance of said
couplings having an axial length of no more than a dimension x,
where X is substantially greater than x, equipped with apparatus
according to claim 9.
17. A structure according to claim 16 being a drill string or
production tubing within an oil or gas well.
18. (canceled)
Description
[0001] The present invention relates to acoustic telemetry and more
particularly to a method of transmitting data acoustically through
tubular structures.
[0002] The invention is especially concerned with the acoustic
transmission of data through long tubular structures of a generally
periodic nature, such as drill strings or production tubing in oil
or gas wells, and oil, water and gas pipelines, which are composed
of many individual tubing sections joined end to end by couplings.
The kind of structure over which the technique of the present
invention is intended to operate will typically comprise at least
ten such tubing sections but there will usually be very many more;
for example it is not uncommon for deep production oil and gas
wells to extend to depths of several kilometres and include
production tubing strings numbering hundreds of individual
sections. There is frequently a requirement for the transmission of
data both downhole and uphole within a well, for example to
transmit command signals from the surface for the operation of
downhole motors, pumps, valves, actuators or other tools, and
information signals to the surface from downhole flowmeters, strain
gauges, temperature and pressure sensors, data loggers etc.
Acoustic techniques where the tubular structure itself acts as a
waveguide for the transmission of signals between different points
along its length have been known for some time, but have hitherto
not been entirely satisfactory in terms of received signal quality
and power consumption, particularly when required to operate over
long distances.
[0003] The invention will be described with reference to the
accompanying drawings, in which:--
[0004] FIG. 1 is a diagram illustrating the generation of multiple
acoustic signal reflections within an individual section of a
periodic tubular structure;
[0005] FIGS. 2 and 3 illustrate typical phase and group velocities
for various acoustic modes within a tubular metal structure;
and
[0006] FIG. 4 illustrates schematically an acoustic telemetry
system according to the invention as installed in a production oil
well.
[0007] One of the problems which is encountered in acoustic
telemetry of the kind indicated above is the interference effect of
so-called Brillouin scattering, which is caused by signal
reflections from the boundaries of the tubing sections and
couplings. Consider for example FIG. 1 which shows a portion of a
long tubular structure comprising many individual tubing sections
T.sub.1, T.sub.2, T.sub.3 . . . T.sub.n joined end to end by
couplings C.sub.1, C.sub.2 . . . C.sub.n-1. In the case of standard
steel oil production pipe the tubing sections are typically 4-23 cm
in diameter and nominally 9-14 m long, screwthreaded at each end
into tubular couplings typically 20-50 cm long. Now consider a
signal S travelling as a guided acoustic wave through the tubular
structure, and the reflections occurring within the illustrated
section T.sub.2. As the signal S travels down through the section
T.sub.2 it meets the boundary with the coupling C.sub.2. While the
transmission and reflection coefficients are determined by the mode
and the boundary geometry, in general for long wavelength modes the
acoustic impedances (not characteristic impedance) of the pipe
section and coupling will be similar and most of the signal energy
passes across this boundary unimpeded. However there is generally
some degree of mismatch between the acoustic impedances and a small
portion of the signal energy is reflected back towards the coupling
C.sub.1, notionally indicated in the Figure as reflection R.sub.1,
propagating as a smaller signal of length initially equal to that
of the signal incident upon the boundary with C.sub.2. At the
boundary with C.sub.1 a portion of the reflected signal R.sub.1
will itself be reflected and propagate as a double-reflected signal
R.sub.2 back down the tubing section T.sub.2 in the same direction
as the signal S. At the boundary with C.sub.2 a portion of signal
R.sub.2 will again be reflected as R.sub.3 and so on with
subsequent reverberations of decreasing energy passing back and
forth along T.sub.2 until the reflected power is eventually
dissipated. In FIG. 1 a total of four subsequent reverberations
R.sub.1 to R.sub.4 are shown for the purposes of illustration
although in practice there will be many more. In general, mode
conversion will also occur, resulting in the transmission and
reflection of several modes.
[0008] Assuming that the spatial length of the signal S is at least
twice the length L of the tubing section T.sub.2 between the
couplings C.sub.1 and C.sub.2, it will be appreciated that at least
part of the second reflected signal R.sub.2, and perhaps of the
fourth reflected signal R.sub.4 and other subsequent even-numbered
reflections depending on the total length of signal S, will pass
along section T.sub.2 in the same direction and at the same time as
part(s) of the signal S that are still passing through that section
(i.e. part(s) of that signal that follow the leading part of length
2 L), and will consequently interfere with that signal. The
wavelengths of the signals will determine the extent to which this
interference is constructive or destructive. Furthermore it will be
appreciated that the same scattering of the signal S will occur in
each of the tubing sections T.sub.1-T.sub.n, resulting in a complex
trail of reverberations following the leading edge of the signal S
along the structure. The effect is of course equivalent whether the
signal S is propagated in the direction indicated in FIG. 1 (i.e.
in the downhole direction in the case of an oil or gas well) or in
the opposite (uphole) direction. Similar scattering effects may
occur within the lengths of the couplings C.sub.1, C.sub.2 etc,
although these tend to be insignificant if the wavelength of the
transmitted signal is long (at least twice the length) in
comparison with the couplings.
[0009] Since wavelength is frequency dependent, the interference
between the signal S and reflected signals within the tubing
sections results in a series of alternating "pass" and "stop" bands
together with a further series of "stop" frequencies (sometimes
referred to as "fine structure" or "comb structure") within each
pass band, the number of "stop" frequencies within the fine
structure of these bands being related to the total number of
tubing sections. Stop frequencies will occur, for example, at
frequencies where the length of a tubular section is equal to half
a wavelength or multiples thereof and pass frequencies will occur
at frequencies where the length of a tubular section is equal to an
odd multiple of a quarter-wavelength (i.e. the frequencies lying
between the half-wavelength stop frequencies).
[0010] This effect has been recognised in the art for some time. It
might therefore be expected that signals could be transmitted with
little attenuation along the length of such a structure simply by
selecting a frequency in a pass band calculated from the nominal
length of the sections from which it is composed. In most practical
cases, however, the tubular sections vary in length at least to
some extent, and sometimes by design. The corresponding stop and
pass bands therefore overlap with each other and a clean signal
cannot be propagated throughout a structure of any significant
length.
[0011] Others have proposed techniques to overcome the difficulties
of acoustic telemetry through structures of this kind. For example
U.S. Pat. No. 5,128,901 proposes a method of acoustic telemetry
through a drill string using a modulated continuous acoustical
carrier wave in the pass bands of the drill string and where the
data signal is preconditioned by multiplying each frequency
component by exp(-ikL) where i is -1, k is the wave number in the
drill string at the frequency of each component and L is the
transmission length of the structure. However this method is still
likely to suffer from mode conversion and interference effects at
the couplings, it is necessary to know both the pass bands and L
with accuracy, and the use of a continuous carrier wave implies
substantial power consumption during operation of the system. U.S.
Pat. No. 6,442,105 proposes an alternative approach, for acoustic
telemetry through oil well production tubing, using a broadband
communications technique where transmitted signals comprise a sweep
of selected frequencies over a time period, i.e. chirp signals, and
which relies on at least one of the frequencies reaching the other
end of the structure. This method is however wasteful of power as
it is expected that a large proportion of the transmitted energy
will be blocked in the course of passage through the structure and
each signal must have a substantial length in order to complete the
frequency sweep. U.S. Pat. No. 5,050,132 proposes a method of
acoustically transmitting data signals over a drillstring which
aims to avoid destructive interference caused by the signal being
reflected back and forth from the ends of the drillstring, by
transmitting in a passband of the drillstring and limiting the time
period of each transmission to be equal to or less than the time
for the data signals to travel three lengths of the drillstring.
However this fails to recognise the Brillouin scattering
interference effect due to signal reflections within the individual
tubing sections, which cannot be overcome solely by addressing
reflections from the ends of the whole structure. Furthermore the
proposed technique will not even prevent interference from being
caused by the signals being reflected back and forth from the ends
of the entire string unless the stated time period is truncated to
the time taken for the data signals to travel only twice the length
of the drillstring.
[0012] It is observed in relation to the prior art techniques
indicated above that, particularly in the case of data transmission
in the uphole direction, a telemetry method which minimises power
consumption is highly desirable as the power available downhole for
operation of the system is likely to be at a premium.
[0013] With reference to the Brillouin scattering problem discussed
above one factor which the prior art has failed to exploit is that,
within a given tubular section, interference of the transmitted
signal with its own reflection(s) only occurs when the signal is of
a spatial length greater than twice the distance of the section
between couplings (or in other words of a temporal length greater
than twice that distance divided by the signal's speed of
travel).
[0014] With the foregoing in mind, in one aspect the present
invention resides in a method of transmitting data acoustically
through a tubular structure predominantly comprising a series of
tubing sections joined end to end by couplings, at least a
preponderance of said tubing sections having an axial length of at
least a dimension X between couplings and at least a preponderance
of said couplings having an axial length of no more than a
dimension x, where X is substantially greater than x; the method
comprising propagating along the structure, from a first position
thereon, acoustic signals in the form of tone bursts at least
predominantly comprising a selected guided wave mode with a
wavelength of at least 2x, and detecting said signals from a second
position on the structure, where the distance N along the structure
between said first and second positions is at least 10X, and
wherein each said burst has, at least as initially propagated, a
temporal length of no more than a multiple of X/C and substantially
less than 2N/C, where C is the phase velocity of the selected mode.
The invention also resides in apparatus for transmitting data in
accordance with such method and in a structure equipped with such
apparatus.
[0015] In this respect a "tone burst" will be understood to mean at
least one, and preferably several, complete cycles of the selected
wave, the maximum available number of cycles in each burst at a
given frequency being determined by the above-defined temporal
length limit.
[0016] From the foregoing discussion of the interference effects of
Brillouin scattering it will be appreciated that the theoretical
ideal solution in a method according to the invention is to apply a
temporal length limit of 2X/C to each transmitted tone burst. If so
truncated, Brillouin scattering is not actually avoided and each
burst as received at the second said position will generally be
followed by a trail of unwanted signals resulting from reflections
and reverberations within the structure. However, limiting the
burst length in this way, effectively to a length which can
generally be received as a "clean" signal undistorted by the
effects of the Brillouin scattering, means that optimal use can be
made of power available at the point of transmission and is not
unduly wasted on signal components that are poorly transmitted
through the structure.
[0017] This also assumes that there is minimal dispersion of the
signal in its passage through the structure so that lengthening of
the signal does not occur to the extent that will lead to
significant attenuation by interfering reflections within
individual tubing sections. If necessary, steps can be taken to
reduce the incidence of dispersive effects, such as by applying a
Hanning window or other pulse shaping envelope to the transmitted
tone burst to suppress the production of side bands. However this
also means that in some circumstances it may actually be preferable
to select an initial temporal length limit of somewhat less than
2X/C.
[0018] On the other hand, there may also be circumstances in which
the benefits of the invention are still realised to a useful extent
where the temporal length of the signal as transmitted is greater
than the theoretical ideal, for example when there is little
variation in individual pipe lengths or other geometrical
conditions are such that the onset of Brillouin scattering effects
and corresponding power wastage is not too severe notwithstanding a
somewhat lengthened signal. Thus in other embodiments the signal
length might be set at, say, 5X/C, 10X/C or up to around 20X/C.
This should also place fewer constraints on the precision of the
associated acoustic transducer design and enable the use of
lower-cost system hardware.
[0019] Coding of data in a method according to the invention can be
by the simple presence or absence of a transmitted burst during
successive time periods (i.e. pulse position coding) or, since it
should generally be possible to discriminate the transmitted tone
bursts from following reverberations, a higher data rate method may
be used, such as frequency or amplitude keying of the bursts. At
the receiving end, signal correlation or other methods generally
known in the art can be used to identify the correct signal. The
temporal spacing between successive bursts should be chosen to
allow the reverberations from the preceding burst to have decayed
to an acceptable level before transmitting the next, in order to
avoid interference. However, the fact that the length of each
transmitted burst is limited in accordance with the invention also
reduces the subsequent reverberation period as compared to known
prior art methods.
[0020] In selecting the guided wave mode for a method according to
the invention it is noted that there are three groups of modes that
will propagate along the length of a tubular structure of the kind
in question, namely flexural, longitudinal and torsional modes. For
the purposes of the present invention it is preferred that the
selected mode has both low surface radial displacement and high
group velocity. The first of these criteria is desirable because
surface radial displacement couples energy to the fluid within
and/or surrounding the structure, resulting in strong damping of
the transmitted signals, while the second facilitates isolation of
the transmitted signals from reverberations and mode-converted
signals which follow them.
[0021] There are an infinite number of modes for a structure of the
kind in question, but the most suitable is believed to be the
L(0,1) or first longitudinal mode, at a frequency at the lower end
of its branch. The useful frequency band for this mode exists from
a lower frequency limited by the length of the shortest tubular
section with respect to the wavelength, up to a higher frequency
limit defined by the acceptable limit of increasing dispersion
governed by the inner and outer diameters of the tubular sections
and the material from which they are made. FIGS. 2 and 3 illustrate
phase and group velocities for various modes modelled for typical
18 cm outside diameter steel oil production pipe with an
approximately 1 cm wall thickness. The dotted modes are flexural
and can be seen to have lower maximum group velocities than the
illustrated longitudinal L(0, 1) and L(0,2) modes over the
illustrated frequency range. The first longitudinal mode can be
seen to extend at a usefully high group velocity from zero
frequency up to around 7.5 kHz which indicates an upper frequency
limit for the telemetry system if it is to operate using this mode,
although the maximum velocity occurs at a substantially lower
frequency and the most preferred operating range is a compromise
between velocity and allowable number of cycles in each tone burst.
The L(0,1) mode at low frequency is preferred over other modes
because it has minimal radial motion at the edges of the pipe wall
over the lower frequency band and should offer the lowest possible
attenuation through leakage into the contacting fluid.
[0022] Although the invention has been described above in relation
to a pipeline with discrete coupling structures C.sub.1 etc. for
joining successive tubing sections, in other kinds of structure to
which the invention is applicable the couplings need not be
separate items from the tubing sections and such sections may be
connected e.g. by respective male and female threaded portions at
opposite ends. The couplings then comprise those lengths of
adjacent sections over which they are screwed together. It may also
be applicable to welded pipe sections or indeed to any long tubular
structure having regular discontinuities in the acoustic path, and
the term "coupling" is to be broadly interpreted accordingly.
[0023] The means for propagating and detecting the acoustic signals
in a method according to the invention may comprise transducers
based on any suitable design principles generally known in the art,
but in view of the short signal lengths required by the invention
they are preferably solid state devices such as transducers
comprising stacks of piezoelectric elements, or magnetostrictive
material, adapted to be clamped or permanently affixed to the
respective tubing sections.
[0024] FIG. 4 illustrates a simple embodiment of an acoustic
telemetry system according to the invention as installed in a
production oil well. Production tubing, comprising numerous tubing
sections 1 joined end to end by couplings 2, extends through the
well inside an outer casing 3 from the traditional well head
structure 4 down to a reservoir of product 5 where the outer casing
is perforated to allow flow into the open end of the lowermost
section 1, and with a packer 6 between the casing and the
production tubing above the reservoir, all as is conventional. By
way of example, a pressure sensor 7 and a flow control valve 8 are
provided towards the lower end of the tubing string and are wired
for communication with an acoustic transducer 9 mounted to the
tubing. At the upper end of the tubing string another acoustic
transducer 10 is mounted to the tubing and wired or otherwise
adapted to communicate with a surface control station (not shown)
via the well head 4. The transducers 9 and 10 communicate by series
of acoustic tone bursts transmitted though the production tubing in
accordance with the method of the invention, e.g. to transmit data
from the sensor 7 to the surface and to transmit control signals
from the surface to the valve 8. The downhole equipment 7, 8, 9 may
be powered for this purpose by batteries or from the surface, but
preferably by means of downhole power harvesting devices which
generate electricity in response to the flow of product though the
tubing string, such as the devices described in our copending
International patent application no. GB2006/004777.
* * * * *